Neuromuscular
Control
1
Motor Units
The motor unit consists of the anterior
motor neuron and the specific muscle
fibers it innervates
Each
muscle fiber generally receives input
from only one neuron
Yet a motor neuron may innervate many
muscle fibers
The number of muscle fibers per motor
neuron generally relates to a muscle’s
particular movement function
Muscles
that require less precision may have
several hundred fibers served by one motor
neuron.
Muscles that function with great precision
may have as few as one muscle fiber per
motor neuron.
Fig. 19.7. McArdle et al. 2015. Exercise
Physiology… LWW
All‐or‐None Principle
A stimulus strong enough to trigger
an action potential in the motor
neuron activates all of the
accompanying muscle fibers in the
motor unit to contract
synchronously
There
is no such thing as a motor neuron
stimulus that causes only some of the
fibers to contract.
A motor unit does not exert a force
gradation; either the impulse elicits
an action or it does not
Similarly, a stronger action potential
cannot produce a stronger contraction.
Fig. 19.7. McArdle et al. 2015. Exercise
Physiology… LWW
Summation of Force
Normal body movements are
not made up of simple twitches,
but sustained contractions.
↑frequency of stimuli increases
> fibers do not have time to
relax > “summation” and ↑
force production
Fig. 1.12,13. Kenney et al.
2012. Physiology of Sport…
Hum.Kin.
Gradation of Force (Acute)
Amount of force generated in a single muscle fiber
is dependent upon the # of crossbridges
Force generation of a group of muscles depends on
four factors:
1)
Number of motor units recruited
• A muscle generates considerable force when activated by all of its
motor units
2)
Frequency of motor unit discharge
• Repetitive stimuli that reach a muscle before it relaxes increases
the total tension
3)
Type of motor unit recruited
• Larger / Type II motor units will generate greater force
4)
Preloading the muscle (activating the stretch reflex)
Types of Muscle Action
Tab. 18.1. Plowman et al. 2014.
Exercise Physiology… LWW
Force‐Velocity Curve
Fig. 1.14. Kenney et
al. 2012. Physiology
of Sport… Hum.Kin.
Force‐Velocity Curve
Fig. 1.14. Kenney et
al. 2012. Physiology
of Sport… Hum.Kin.
Figure & Notes References
McCardle, Katch, Katch. Exercise Physiology:
Nutrition, Energy, and Human Performance, 8th
Edition. Wolters Kluwer Health, 2014.
Kenney, Wilmore, Costill. Physiology of Sport and
Exercise (5th ed). Human Kinetics, 2011.
Plowman SA & Smith DL. Exercise Physiology for
Health, Fitness, & Performance (4th ed).
Lippincott Williams & Wilkins, 2014
Motor Unit Types
1
Motor Unit Characteristics
A motor unit contains only one specific muscle
fiber type (type I or type II) or a subdivision of
the type II fiber with the same metabolic
profile
Motor units are classified based on three
physiologic and mechanical properties of the
muscle fibers they innervate:
Twitch
characteristics
Tension characteristics
Fatigability
2
Twitch Characteristics
Major characteristics for common motor unit
categories:
Fast
twitch, high force, and fast fatigue (type IIx)
Fast twitch, moderate force, and fatigue resistant
(type IIa)
Slow twitch, low force, and fatigue resistant (type I)
Motor neurons have a stimulating effect on
the muscle fibers they innervate in a way that
modulates the fibers’ properties and adaptive
response to stimuli
3
Motor Unit Characteristics
Type IIx
Fig. 19.11. McArdle et al. 2010.
Exercise Physiology… LWW
Type IIa
Type I
4
Motor Unit Characteristics
>300
Tab. 1.1,2. Kenney et al. 2012.
Physiology of Sport… Hum.Kin.
5
Size Principle
Low‐threshold motor units
are recruited first and have
lower force capabilities
than higher‐threshold
motor units.
Typically, to get to the high‐
threshold motor units, the
body must first recruit the
lower‐threshold motor
units.
Fig. 5.2. Baechle et al. 2009. Essentials
of Strength… Human Kinetics
6
Size Principle
This orderly recruitment of
specific motor units to produce
a smooth muscle action allows
the CNS to fine tune skeletal
muscle activity to meet
demands of the motor task.
Exceptions exist, especially
with respect to explosive,
ballistic contractions that can
selectively recruit high‐
threshold units to rapidly
achieve more force and power.
(Selective recruitment)
Fig. 19.12. McArdle et al. 2015.
Exercise
7
Physiology… LWW
Figure & Notes References
McCardle, Katch, Katch. Exercise Physiology:
Nutrition, Energy, and Human Performance, 8th
Edition. Wolters Kluwer Health, 2014.
Baechle & Earle. Essentials of Strength &
Conditioning, 3rd Editiion. Human Kinetics, 2009.
Kenney, Wilmore, Costill. Physiology of Sport and
Exercise (5th ed). Human Kinetics, 2011.
8
Proprioceptors
1
Proprioceptors
Sensory receptors in the muscles and tendons
that are sensitive to stretch, tension, and pressure
Ex.
Muscles spindles and golgi tendon organ
Almost instantaneously relay information about
muscular dynamics and limb movement to
conscious and subconscious portions of the CNS
Allows continual monitoring of the progress of any
sequence of movements and serves to modify
subsequent motor behavior
Important
for powerful, complex coordinated movements
Can enhance muscle relaxation
https://www.youtube.com/watch?v=7T4NI_2qDEM
2
Muscle Spindles
Provide mechano‐sensory information about changes
in muscle fiber length and tension
Primarily respond to any stretch of a muscle and
initiate a stronger muscle action to counteract this
stretch
Important to utilize for powerful
contractions
Important to minimize when
relaxing a muscle
Fig. 19.13. McArdle et al. 2010.
Exercise Physiology… LWW
3
Neurophysiological Model of
Stretch Reflex
Activation of muscle spindles
initiates a stretch reflex
Known
as “pre‐loading” the
muscle
Three main components:
Muscle
spindle that responds to
stretch
Afferent nerve fiber that carries
the sensory impulse from the
spindle to the spinal cord
Efferent spinal cord motor
neuron that activates the
stretched muscle fibers
Ex. patellar reflex
Fig. 19.14. McArdle et al. 2010.
Exercise Physiology… LWW
4
Mechanical Model of
Stretch Reflex
Elastic energy in tendons and muscles is
increased with a rapid stretch (eccentric
muscle action)
and then briefly stored.
If a concentric muscle action
follows immediately, the stored energy is
released, contributing to the total force
production.
If a concentric muscle action does not
occur immediately following the eccentric
action, then the stored energy dissipates
and is lost as heat.
5
Plyometric Mechanics and the
Stretch Shortening Cycle
The stretch‐shortening cycle (SSC) combines
mechanical and neurophysiological mechanisms and
is the basis of plyometric exercise.
A rapid eccentric muscle action stimulates the
stretch reflex and storage of elastic energy, which
increase the force produced during the subsequent
concentric action.
Table 16.1. Baechle et al. 2009. Essentials of Strength… Human Kinetics
6
Flexibility and Stretch Reflex
Proprioceptors and Stretching
Static
Dynamic
Ballistic
•
Fig. 13.7. Baechle et al. 2009. Essentials
of Strength… Human Kinetics
A stretch reflex occurs when
muscle spindles are
stimulated during a rapid
stretching movement.
•Limits muscle relaxation
Proprioceptive
Neuromuscular
Facilitation (PNF)
Stretching
7
Golgi Tendon Organs
Connect to extrafusal fibers
near tendon’s junction to
muscle
Detect differences in the
tension generated by active
muscle to protect the muscle
and surrounding connective
tissue harness from injury from
sudden or excessive load
GTO
activation directly related to
load applied
When stimulated, Golgi
receptors transmit signals to the
spinal cord to elicit reflex
inhibition of the muscles they
supply
Fig. 19.15. McArdle et al. 2010.
Exercise Physiology… LWW
8
GTO Inhibition & Stretching
Proprioceptors and Stretching
Fig. 13.7. Baechle et al. 2009. Essentials
of Strength… Human Kinetics
Proprioceptive
Neuromuscular
Facilitation (PNF) Stretching
•Autogenic inhibition is
accomplished via active contraction
before a passive stretch of the same
muscle.
•Reciprocal inhibition is
accomplished by contracting the
muscle opposing the muscle that is
being passively stretched.
•Both result from stimulation of
Golgi tendon organs, which cause
reflexive muscle relaxation.
9
PNF Hamstring Stretches
Hold‐Relax (“Push & hold”)
Passive
prestretch (10 seconds), isometric hold
(6 seconds), passive stretch (30 seconds)
https://www.youtube.com/watch?v=gvOQK8qNi9E
Fig. 13.1,2. Baechle et al. 2009. Essentials of Strength… Human Kinetics
10
PNF Hamstring Stretches
Hold‐Relax (“Push & hold”)
Passive
prestretch (10 seconds), isometric hold
(6 seconds), passive stretch (30 seconds)
Contract‐Relax (“Move”)
Passive
prestretch (10 seconds), concentric muscle action
through full ROM, passive stretch (30 seconds)
https://www.youtube.com/watch?v=gvOQK8qNi9E
Fig. 13.1,2. Baechle et al. 2009. Essentials of Strength… Human Kinetics
11
PNF Hamstring Stretches
Hold‐Relax With Agonist Contraction
(“Push,
then Pull”)
During
third phase (passive stretch), concentric action of
the agonist used to increase the stretch force
Fig. 13.10,11. Baechle et al. 2009. Essentials of Strength… Human Kinetics
12
Figure & Notes References
McCardle, Katch, Katch. Exercise Physiology:
Nutrition, Energy, and Human Performance, 8th
Edition. Wolters Kluwer Health, 2014.
Baechle & Earle. Essentials of Strength &
Conditioning, 3rd Editiion. Human Kinetics, 2009.
13
EXNS 6203 – Guided Study Worksheet
Name:
Week 11 – Neuromuscular Control
1) Explain how the All or None Principle dictates that individual muscle fibers cannot exert a
gradation of force. Then explain how muscle groups are able exert a gradation of force.
2) Explain the force-velocity curve and how it applies to exercise training.
3) Compare and contrast Type 1 and Type 2 motor units. Use the Size Principle in part of your
explanation.
4) Distinguish between the influence of muscle spindles and golgi tendon organs on muscle
contraction.
5) Explain the sequence of events involved in preloading a muscle. What is the purpose of
preloading? How does plyometric training improve your stretch reflex?
6) Describe the role of the GTOs in PNF stretching.
Purchase answer to see full
attachment